In order to diagnose an abnormality inside an oil-filled electrical apparatus, analysis of a gas in an oil, by which a diagnosis can be made without stopping the apparatus, is used at home and abroad (for example, PTD 1: Japanese Patent Laying-Open No. 2004-200348, and NPD 1: Z. Wang, X. Wang, X. Yi and S. Li, “Gas Generation in Natural Ester and Mineral Oil Under Partial Discharge and Sparking Faults”, IEEE Electrical Insulation Magazine, Vol. 29, No. 5, pp. 62-70, 2013). In order to determine whether or not there is an internal abnormality and to determine the type (cause) thereof, types of gas components contained in an oil, concentrations thereof in the oil, a concentration ratio therebetween, and the like are used as parameters.
As a method for diagnosing whether or not there is an abnormality caused by discharge (discharge abnormality), which is a type of internal abnormality, diagnostic methods using hydrogen gas and acetylene gas as parameters are known. Compared with acetylene gas, hydrogen gas is generated at a lower temperature and in a larger amount. Thus, the diagnostic method using hydrogen gas as a parameter can diagnose a discharge abnormality earlier than a diagnostic method not using hydrogen gas as a parameter.